The present invention relates to devices for measuring the flow of fluid through conduits. In particular, the present invention relates to instruments of the type generally referred to as flowmeters.
Such flowmeters may be of either (1) the noninvasive type, which do not interrupt the fluid flow, but rather establish a field or signal within the conduit which is varied in response to the fluid flow, or (2) the invasive type in which an element is disposed in the flow path for measuring the fluid flow. The present invention relates to the latter invasive type of flowmeter and, in particular, to a flowmeter of the type which utilizes an impeller which is rotated by the fluid at a rate which is proportional to the fluid velocity in the conduit.
Many types of impeller flowmeters are known, most being of the turbine type which utilize a rotating turbine mounted coaxially in the conduit. Such turbine flowmeters, however, do not operate well in anything except clean fluids, since any type of particulate matter in the fluid tends to clog the turbine or impair the free rotation thereof, resulting in inaccurate measurements. Furthermore, the turbine is subject to considerable wear and damage, since it is completely exposed at all times to the fluid and any particles or objects which may be carried therein.
It is known to use paddle wheel type impellers which project laterally into the fluid stream, and this alleviates the clogging problem and permits the use of this type of impeller with fluids other than completely clean fluids. However, prior paddle wheel impellers are still subject to considerable mechanical damage.
Furthermore, most prior impeller type flowmeters utilize magnetic or optical pickups to sense the rotational speed of the impeller. Such magnetic pickups create a drag on the impeller, thereby affecting the accuracy of the readings obtained. They also collect rust particles which tend to clog the apparatus. Optical pickups have the disadvantage of being usable only in clear fluids. They will not work, e.g., in frothy or foamy liquids.
Finally, prior paddle wheel flowmeters are mounted in special housings which must be inserted as a separate section in the fluid conduit and which necessitate changing the direction of fluid flow.